1. Field of the Invention
The present invention relates to an element analyzing method using fluorescent X-rays, and more particularly to an element analyzing method which makes possible precise identification of light elements and precise calculation of concentrations thereof.
2. Related Background Art
The X-ray fluorescence analysis has been heretofore used as a nondestructive element analysis for an object to be measured (sample). The total reflection X-ray fluorescence analysis was developed to improve the sensitivity and is under study for applications to contamination control in semiconductor processes. Among the total reflection X-ray fluorescence analyses, the energy dispersive X-ray fluorescence analysis permits measurement of spectrums in a wide range of energy, so that multiple elements can be simultaneously analyzed thereby by using a single solid-state detector (SSD) disposed immediately above the sample. Since the energy dispersive X-ray fluorescence analysis needs no analyzing crystal, the SSD can be set closer to the sample. Therefore, the energy dispersive method has the feature of higher sensitivity for example than the wave dispersive method in X-ray fluorescence analysis.
The energy-dispersive total reflection X-ray fluorescence analysis is inferior in spectrum resolution to the wave length dispersive method, providing a smaller total count number. Also, the total reflection method tends to have impeding peaks, for example escape peaks, peaks due to influence of diffraction beams, sum peaks (2K.sub..alpha. ; K.sub..alpha. +K.sub..beta.), peaks due to the Compton scattering, and so on.
The conventional energy dispersive X-ray fluorescence analysis permits easy discrimination between element peaks and impeding peaks if concentrations of contaminating elements are high. It is, however, difficult to discriminate light elements in concentrations of not more than 10.sup.9 atoms/cm.sup.2, which is required in semiconductor processes, by using the conventional energy dispersive method. Reasons for this difficulty are as follows.
(1) The generation probability (fluorescence yield) of the K.sub..alpha. spectrum is for example 0.02 for Na, which is 1/20 of 0.4 for Zn. Thus, even if light elements (Na to Cl) exist in a sample in the same concentration as heavy elements, a total count number (K.sub..alpha. fluorescent line) obtained for each light element is smaller than those for heavy elements.
(2) A beryllium (Be) window is set at the fore end of the solid-state detector to maintain a high vacuum inside the detector and to prevent contamination on the surface of a Si crystal, of which the solid-state detector is made. The Be window absorbs the K.sub..alpha. spectrum. That is, if the Be window has a thickness of about 10 .mu.m, a light element lighter than F cannot be detected because of the absorption of the K.sub.60 line. Elements above Cl are free of the influence of absorption of the K.sub..alpha. line. Further, elements between F and Cl are influenced by the absorption depending upon the mass absorption coefficient thereof.
(3) Since a sample is made mainly of Si in a semiconductor process, the resultant spectrum will include peaks of Na to Cl elements on the tail of the K.sub..alpha. spectrum of the Si element. Thus, some means is required for making the half width of the Si peak as narrow as possible, for example, means for setting the X-ray incident angle amply smaller than the critical angle. This technique, however, decreases the total count number obtained, resulting in being readily affected by statistical error of counting.